Tighten common code validation and group selection

TaxBaik.Web/Components/Admin/Shared/CommonCodeGroupPanel.razor

@@ -5,7 +5,9 @@
                    ValueChanged="OnSelectedGroupChanged"
                    Label="코드 그룹"
                    Variant="Variant.Outlined"
-                   FullWidth="true">
+                   FullWidth="true"
+                   Clearable="true">
+            <MudSelectItem Value="@string.Empty">선택</MudSelectItem>
             @foreach (var group in Groups)
             {
                 <MudSelectItem Value="@group">@group</MudSelectItem>

TaxBaik.Web/Controllers/CommonCodeController.cs

@@ -64,6 +64,8 @@ public class CommonCodeController(CommonCodeService commonCodeService) : Control
     {
         if (string.IsNullOrWhiteSpace(code.CodeGroup) || string.IsNullOrWhiteSpace(code.CodeValue) || string.IsNullOrWhiteSpace(code.CodeName))
             return BadRequest(new { error = "코드 그룹, 값, 이름은 필수입니다." });
+        if (code.CodeGroup.Any(char.IsWhiteSpace))
+            return BadRequest(new { error = "code_group에는 공백을 사용할 수 없습니다." });
         if (code.CodeValue.Contains(' '))
             return BadRequest(new { error = "code_value에는 공백을 사용할 수 없습니다." });